The water-absorbing resin has been utilized widely in hygiene products such as disposable diapers, sanitary napkins and incontinent pads for adults, and various applications such as a water retention agent for soil, due to property of absorbing aqueous liquid in a large amount of from several times to several hundred times of own weight, and has been produced and consumed in a large amount. Such a water-absorbing resin (which is also referred to as a high water-absorbing resin, or a water-absorbing polymer) is described in Japanese Industrial Standard (JIS) K7223-1996, or introduced in many commercially available reference books.
In recent years, in applications to hygiene products such as, in particular, disposable diapers, sanitary napkins, incontinent pads for adults, there has been tendency to increase use amount of the water-absorbing resin and decrease use amount of pulp fiber, to make a thin product. Due to this tendency, it is necessary for the water-absorbing resin to bear a part of function of liquid permeation or diffusion conventionally performed by pulp in an absorbing body. As a known index for evaluating superiority or inferiority of such a function, absorbency or liquid permeability of the water-absorbing resin under pressure has been proposed, and those having higher value thereof have been desired. Further, this tendency of a thinner type also leads to increase in use amount of the water-absorbing resin per one piece of the hygiene product, therefore, request for a low cost water-absorbing resin has been increasing.
In general, the water-absorbing resin is obtained by obtaining a water-containing polymer by aqueous polymerization of a hydrophilic unsaturated monomer, and then by drying, pulverizing and classifying this. This is referred to as a base polymer in the present application. As the water-absorbing resin for application to hygiene products, usually the base polymer subjected to surface treatment is used. Surface treatment is performed generally by mixing a surface cross-linking agent solution to the base polymer and then by performing heat treatment. The present inventors have proposed a way for solving a problem in a surface treatment step, in our previous applications (refer to Patent Documents 1 to 4), however, the present application is one still more deepened and developed this. For example, in Patent Document 5, there has been disclosed a method for spraying a surface cross-linking agent solution in a hollow cone shape, or an ellipse cone shape showing both convex lens shape, under atmosphere with a dew point of equal to or lower than 60° C. and a temperature of equal to or higher than 90° C. to perform heat treatment of the water-absorbing resin.